The present invention relates to a roll assembly for a fiber-web having at least at one end thereof a bearing assembly with a bearing housing, which bearing housing comprises a flexible and attenuating attachment arrangement, by means of which the roll can be attached in a flexible and attenuating manner on the frame or foundation of the fiber-web machine. The invention also relates to a method of attenuating the vibration of a fiber-web machine roll, in which method the roll is rotatably attached at its ends on a bearing assembly in bearing housings, which bearing housings are supported in a flexible and attenuating manner on the frame or foundation of the fiber-web machine.
Along with the increasing widths and higher speeds of fiber-web machines, the vibration of rolls is becoming an ever-increasing problem. The rolls in fiber-web machines are used, for instance, to guide the travel of the web and/or support fabric. As for the properties of the fibrous web, such operative positions of the rolls, in which the roll is involved in the processing of the web, such as roll nips, calender rolls, counter rolls for coating and winding drums or reeling drums are, however, the more essential. Especially, the winding drums and drums of slitter-winders used for web finishing are challenging objects in terms of vibration, since the winding of customer reels of different sizes and possibly containing various kinds of flaws will easily get disturbed due to excessive vibration. Traditionally, the rolls are attached onto foundations as rigidly as possible, especially when the drums of a slitter-winder are concerned.
Vibration is a problem particularly in two-drum or belt-type winders. A vibration problem occurring with drum winders arises when the harmonics of the rotational speed of a paper roll produced on drums excites the natural frequencies of the drums. The same type of a vibration problem occurs also with the reeling drums of reel-ups. The resonance vibrations during the operation of a machine or a device are often caused by insufficient damping, i.e. inadequate dynamic stiffness at the resonance frequency. It is often possible to improve the situation by directly modifying the resonating structure in order to increase its damping.
Publications WO9849394, WO0004227 and WO0004228 disclose solutions, in which an adjustable mass absorber is arranged in the bearing housing of a paper machine roll. In the solution according to the publications, the adjustability of the frequency of the vibration to be absorbed is realized by changing the position of the mass on a vibrating rod or alternatively, by changing the magnitude of the mass. Since the absorber is relatively easy to access, the tuning can be changed even during the operation of the machine. However, this kind of a solution is not always capable of providing sufficient attenuation of vibration. To provide efficient attenuation of vibration the vibrating mass should be very large, which is not advantageous e.g. in view of space utilization and also due to the solid support structures, which are then required.
F1 patent 94458 discloses a method and an apparatus for controlling the vibrations of paper machine rolls. According to the method, the locations of critical speed areas are changed during the operation. The critical speed is changed by adjusting the mass and/or the stiffness of the roll, and/or the attachment point of the roll. Amending the stiffness of the bearing assembly at the ends of the roll is suggested as an alternative. Intermediate pieces of elastic material may be placed between the base plate of the bearing housings of the end bearings and the frame. The stiffness of the attachment of the bearing housings can be adjusted by adjusting the force, by which the bearing housing presses the intermediate pieces against the frame. Said pressing force can be adjusted by means of a cylinder device or a screw.
JP patent publication 3082843 discloses an arrangement for attenuating vibrations of a roll. The drive motor of the roll is flexibly attached to the frame. The attachment includes a vibration-proof intermediate piece of rubber between the bottom plate of the securing part of the drive motor and the frame. The securing bolts of the bottom plate extend through the frame plate to a cylinder attached to the bottom surface of the frame plate, where they are secured to a piston in the cylinder. There are rubber sleeves under the heads of the securing bolts, whereby the attachment of the bottom plate is made floating. The inner surface of the cylinder is provided with an extension, which limits the movement of the piston upwards in the cylinder. There is a spring between the cylinder top and the upper surface of the piston, and a pressure space with pressurized air as the pressure medium between the bottom surface of the piston and the bottom of the cylinder. At first, the piston is taken pneumatically against the extension of the inner surface of the cylinder, whereby the intermediate rubber pieces and the sleeves are subjected to a minimum compression force. As soon as the pressure of the compressed air under the piston is decreased, the piston moves downwards by the force of the spring above the piston, whereby a larger compression force is exerted on the intermediate rubber pieces and the rubber sleeves. Thus, the stiffness of the roll attachment can be regulated by the pressure of the pressure medium under the piston.
F1 patent No. 118482 discloses a roll attachment in a paper or board machine, where the roll is rotatably attached at its ends on bearings in bearing housings, which bearing housings are supported on the frame or foundation of the machine by means of elastic intermediate pieces. In the publication, a specific advantageous range of variation is given to the spring constant of the intermediate pieces. In this arrangement, the damping capacity of a weakly dampened flexible structure is increased by changing the boundary conditions for the structure so that damping is introduced into the structure to be dampened through its attachment. Then, the static stiffness of the structure and its attachment is decreased, but the dynamic stiffness of the structure itself increases. Although the solution as such is advantageous and efficient, there has lately arisen a need to further improve the damping efficiency in order to provide enhanced vibration damping properties.
Slender rolls, in particular, bend more at their lowest natural frequency than the short and thick rolls. Thus, the relative movement between the bearing housing and the roll center is smaller with slender rolls. This weakens the efficiency of the damping solution according to F1 patent 118482 especially for slender rolls. The lowest natural frequency of slender rolls is even with a fixed bearing housing fairly close to the maximum natural frequency of the roll. Hereby, the optimum flexibility of the solution according to F1 118482 cannot necessarily be utilized, as the natural frequency of the roll may drop very close to the maximum rotational frequency of the roll.